Coated papers, which have a coating layer comprising pigment and binder, are used as high-quality printing papers. As such, in addition to printability in terms of ink absorbency and strength of coating layer, it is also important for coated papers to achieve sufficient gloss on the surface of coating layer. However, pressure-smoothing the surface of coating layer for the purpose of achieving higher gloss will inevitably crush the voids in the coating layer and thereby reduce the paper's ink-absorbing capability. In addition, the use as a pigment binder of a large amount of water-soluble or water-dispersant polymer substance, such as copolymer latex, will improve the strength and gloss of the coating layer, but it will also decrease the voids in the coating layer and thereby reduce the paper's ink-absorbing capability. In this sense, gloss and printability are mutually exclusive. Accordingly, with coated papers the types and blending ratios of pigment and adhesive, weight of coating material, degree of smoothing and other factors are determined in a manner achieving an optimal balance between gloss and printability. However, different technologies are needed to obtain high-gloss paper offering excellent printability. In general, the gloss of coated printing paper improves in the order of fine-coated paper, coated paper, art paper, super art paper and cast-coated paper, with cast-coated paper offering the highest gloss. In the present invention, the term “high gloss” means a level of gloss equivalent to or better than that of super art paper. Therefore, high-gloss paper means a coated printing paper having a gloss equivalent to or better than that of super art paper.
The conventional production methods of high-gloss paper include one using a cast coater. Under this method, a wet coating layer comprising pigment and binder is pressure-bonded using a mirrored cast drum and then heated and dried. This method has a drawback in that the production speed is reduced significantly compared with general art paper, coated paper and fine-coated paper.
A production method using a thermal calender, instead of a cast drum, is also known. For example, Japanese Patent Application Laid-open No. 56-68188 and Japanese Patent Publication Nos. 64-10638 and 64-11758 describe methods to produce a coating layer by mixing pigment with polymer latex or water-soluble polymer resin, applying and drying the coating layer, and then treating the coating layer by heat calendering. In these published technologies, a polymer latex with a glass-transition temperature of 5° C., or 38° C. or above, is applied on a support material, and the obtained coating layer is treated with a thermal calender at a temperature above the glass-transition temperature of the latex used. This provides a simple and productive method that is suitable for producing normal coated paper. However, the method results in an insufficient gloss inferior to that of cast-coated paper and even art paper. With thermal calendering, therefore, a level of gloss comparable to that of cast-coated paper cannot be achieved.
A yet another method is the one described in Japanese Patent Application Laid-open No. 59-22683. Under this method, two or more copolymer latexes of different minimum film-formation temperatures are applied on a sheet or sheet having a pigment coating layer and then dried, after which calendering is applied as necessary to smoothen the surface. When dried, the latexes of different minimum film-formation temperatures will generate minute cracks on the surface of coated paper, thereby achieving good ink absorbency without reducing gloss. The key point of this technology is to generate minute cracks on the surface of coated paper, and the drying condition must be given due attention in order to achieve this effect. In other words, the drying condition must be set so that the latex of the lower minimum film-formation temperature will melt completely while that of the higher minimum film-formation temperature will melt only partially. However, it is a common knowledge that drying conditions are generally subject to fluctuations by a number of factors. When it comes to potential industrial applications of this technology, it is virtually impossible to maintain a uniform, constant drying condition throughout the production process. Therefore, under this technology it is extremely difficult to produce papers of stable quality.
As disclosed in Japanese Patent Application Laid-open Nos. 3-167396 and 8-13390, the inventors found that a gloss equivalent to that of coated paper can be achieved without smoothing and that excellent printability in terms of ink absorbency and surface strength can also be achieved, by designing a coated printing paper that comprises a base material having a support material and a pigment coating layer applied on top, wherein the pigment coating layer has a surface layer made of a thermoplastic polymer (emulsion comprising a polymer or copolymer exhibiting thermoplasticity) with a second order transition temperature of 80° C.; and the inventors also found that by calendering the aforementioned surface layer at temperatures not exceeding the second order transition temperature of copolymer latex a high gloss equivalent to or better than that of super art paper can be achieved, together with sufficiently practical levels of printability in terms of ink absorbency, surface strength and dot-error ratio, while eliminating deposits on the calender rolls and thereby attaining higher productivity and manufacturing efficiency. However, these methods had problems in sheet-feed press, such as mottled ink impression, poor printability and insufficient separation from the calender rolls.